Single cavity Fabry-Perot (FP) tunable filters are commonly used in spectral monitoring applications. High finesse devices have impulse-like spectral filter functions that can be scanned across a band of interest in order to determine the spectral optical energy distribution.
One of the most common applications for FP filters is in wavelength division multiplexing (WDM) systems. In commercially available WDM systems, the channel assignments/spacings can be tight, 100 GigaHertz (GHz) to 50 GHz, based on the International Telecommunication Union (ITU) grid. Further, the number of potential channels, channel slots, in a link can be large. Observation of the ITU Grid suggests 100""s of channels per link in the Lxcex1, Cxcex1, and Sxcex1, bands that stretch from about 1491 nanometers (nm) or 200 Terahertz (THz) to about 1612 nm or 186 THz. Additional channels in this range are provided by the 50 GHz offset of the Lxcex2, Cxcex2, and Sxcex2 bands. Still other systems are being proposed that have assignments/spacings in the 10 to 20 GHz range. In each of these systems, the channels are confined to their channel slot or frequency assignment to an absolute accuracy of less than 10 GHz, in some cases. In order to verify the proper operation of these WDM systems, FP-based optical channel monitors are required with pass bands of 10 GHz and less.
When even sharper filter functions, i.e., smaller pass bands, are required, multiple Fabry-Perot filters can be deployed in a cascade configuration. Two cascaded filters effectively double the sharpness of the net filter function. Moreover, careful co-design of the two filter cavities can yield substantial improvements in the side mode suppression.
Expanding the applications for FP tunable filters, beyond the standard monitoring applications or to applications requiring narrowed passbands, requires effort in the design of a class of FP filters called multi-cavity FP tunable filters. These filters have multiple discrete coupled optical cavities. Selection of the mirror reflectivities for the end mirrors and the mirror separating the cavities, along with control over the cavity lengths, leads to the ability to provide filters that have controllable passband profiles during the design stage and dynamically during operation. Most commonly, the filters are designed to have a top-hat pass band profile, which can be used to selectively route single channels or blocks of contiguous channels in a fully or partially populated WDM signal.
Critical to the deployment of multi-cavity FP filters is the fabrication of microelectromechanical system (MEMS)/micro-optical electromechanical system (MOEMS) filter designs. In the past, macro-scale multi-cavity FP filters have been manufactured.
Generally, however, these devices do not have the form factor required for communications applications. Moreover, they typically lack mechanical robustness and have poorer performance. Smaller optical fiber-based multi-cavity FP filters have been proposed for communications applications. The drawbacks here are associated with the difficulties in depositing highly reflecting HR dielectric mirror coatings on the fiber ends and control of other cavity parameters such as end-mirror curvatures. Moreover, fiber-based cavity FP filters typically use piezoelectric-based actuators, which typically suffer from electromechanical instability.
The present invention is directed to a multi-cavity micro-optical Fabry-Perot filter. It uses electrostatically deflected MEMS membranes. Such devices can have excellent mechanical/optical characteristics.
In general, according to one aspect, the invention features a multi-cavity Fabry-Perot filter. The filter comprises a first electrostatically deflectable membrane device. A curved mirror structure is formed on its optical membrane. Similarly, a second electro-statically deflectable membrane device is provided, which has a second curved mirror structure on the membrane. The spacer is used to separate the first membrane device from the second membrane device. The spacer supports a mirror between the first and second curved mirror structures.
According to one embodiment, the first and second membrane devices each comprise a support, a device layer in which a deflectable membrane is formed, and a sacrificial layer, which separates the support from the device layer. The sacrificial layer is selectively removed to release the membrane.
An optically curved surface is formed on the deflectable membrane. Typically, the optical surface is a concave mirror with a continuous surface profile. In alternative embodiments, however, diffractive or Fresnel-type mirror profiles can be used. In order to provide the mirror structures, an optical coating is deposited on the optically curved surfaces of the membrane devices. The optical coating is typically a multi-layer dielectric mirror.
According to other aspects of the present embodiment, the spacer comprises two spacer layers between which a dielectric mirror layer is located. Regions of these first and second spacer layers are preferably removed surrounding an optical axis to thereby expose the dielectric mirror layer of the flat mirror to the device""s optical cavities. This produces a suspended dielectric mirror in a region surrounding the optical axis of the filter.
In some embodiments, a support frame is provided around the spacer. This support frame is preferably integral with the spacer and defines one or two blind holes into which one or both of the membrane devices are installed. Thus, each membrane device is aligned using preferably lithographically-formed features of the support frame/spacer to thereby align the membrane devices relative to each other. In one embodiment, registration features are provided in at least one or both of the blind holes. The membrane devices are abutted against these features to yield a robust alignment system. Spring or biasing elements can also be fashioned in the support frame/spacer to ensure good abutment of the devices against these registration features.
In general, according to another aspect, the invention features a process for assembling multi-cavity Fabry-Perot filters. Generally, this process relies on the production of a precursor structure that will be subsequently singulated, by die sawing for example, into singulated filters. Typically, a first die of electrostatically membrane devices is attached to a first side of a spacer, which includes the flat mirror, followed by the attachment of a second die of electrostatically deflectable membrane devices to a second side of the spacer.
Preferably, to facilitate the die saw singulation, a perimeter around the membranes of the membrane devices is preferably sealed during the attachment or subsequent to the attachment of the dies to the spacer. Further, optical ports on the backsides of the dies are also preferably filled prior to the die saw operation. A good candidate for the fill material is a photoresist, which is later removed, after the sawing is complete, and any particulate matter washed away.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.